Absent a 6 nm to 10 nm oxide coating, it is difficult to synthesize nanoparticles of highly reactive metals, such as aluminum. The reactivity of these metals inhibits nanoparticle isolation and passivation.
Currently, aluminum nanomaterials are prepared using various methods, such as ball-milling, electrical exploding wires, condensation from vapor, catalyzed decomposition of AlH3 and chemical reactions of AlCl3 with LiAlH4. In general, these methods produce low purity nanoparticles. Furthermore, the complexity of these conventional synthesis methods precludes large scale industrial nanoparticle production.
For example, Rieke, R. D. and Chao, L., Syn. React. Inorg. Met. Org. Chem. 1974, 4, 101 and Purdy, Andrew A., et al., “Aluminum Nanoparticle Synthesis by Reduction of Halides with Na/K,” Mater. Res. Soc. Symp. Proc., 2008, vol. 1056, HH03-18 each teach methods for reducing aluminum halides using a sodium potassium alloy under sonication to produce small nanoparticles. These sodium potassium compounds, however, are ineffective for reducing many aluminum compounds such as amides. However, it is difficult to remove potassium salts, such as KCl and KBr, and other byproducts of the reduction reaction from the resulting aluminum nanoparticles, thereby producing salt-contaminated aluminum nanoparticles, or making the isolation of pure nanoparticles excessively expensive, time consuming, and non-scalable. Additionally, passivation of the resultant aluminum nanoparticles using glycerol or in-situ generated fluorocarbon shells is only partially effective.
Purdy, Andrew A., et al., “Aluminum Nanoparticle Synthesis by Reduction of Halides with Na/K,” Mater. Res. Soc. Symp. Proc., vol. 1056, 2008, HH03-18 further teaches a method for producing aluminum nanoparticles by reducing aluminum amide, Al(N(SiMe3)2)3, using lithium powder. The article, however, only contemplates nanoparticle production by the reduction of aluminum amides, rather than aluminum halides, using lithium powders. Evaluating the effectiveness of the lithium for reducing other reactive metal complexes, it was determined that lithium is unable to produce aluminum nanoparticles when reacted with aluminum butoxide.
Other publications, such as Haber, Joel and Buhro, William, “Kinetic Instability of nanocrystalline Aluminum Prepared by Chemical Synthesis; Facile Room-Temperature Grain Growth,” J. Am. Chem. Soc., vol. 120, 1998, 10847 teach methods for producing aluminum nanoparticles by reacting AlCl3 with LiAlH4 and subsequently removing LiCl byproduct with MeOH. This process involves the generation and subsequent decomposition of alane, AlH3, to aluminum nanoparticles and hydrogen. Also, the LiCl byproduct is difficult to remove by the methods used in the article since MeOH reacts with aluminum, and results in low purity aluminum nanocrystalline products.
Therefore there is a need to develop an effective synthesis method for producing pure nanoparticles of reactive metals that addresses the deficiencies of the prior art.